In an optical disk, an error tends to occur in reproduced data due to a defect in a disk substrate or recording layer and dust and scratches on the surface of a disk substrate. In particular, in recent years, since the recording density has become higher and since the thickness of the disk substrate has become as thin as 0.6 mm, the above-mentioned defect, dust, scratches, etc. are more likely to cause an error.
For this reason, in the optical disk recording and reproducing apparatus, there have been high demands for those apparatuses which have a superior correcting capability in the error-correction encoding process in which an error in data reproduced at the time of disk playback are detected and the erroneous data is restored to correct data. Here, an encoding system in which codes having a great code distance are encoded by doubly combining them has been adopted.
Referring to FIG. 8, an explanation will be given of an example of a recording method for an optical disk recording and reproducing apparatus which includes the above-mentioned double error-correction encoding process. Additionally, this technique is disclosed in Japanese Laid-Open Patent Application No. 293161/1996 (Tokukaihei 8-293161 (published on Nov. 5, 1996).
Main data inputted from a host apparatus in a time sequential manner are divided into 128-byte units, to these further added addition data of two bytes, and these are arranged in 128 rows independently. Data of one byte on the same position (that is, on the same column) are collected from 130-byte data in each row so that a first encoding parity of 14 bytes is added thereto. The first encoding parity is arranged along the direction of arrow Q. The first encoding parity thus arranged consists of 14 rows each having 130 bytes. A second coding parity of 8 bytes is added to each column of 142 rows each having 130 bytes thus arranged.
In this manner, to the appendage data of 2×128 bytes and the main data of 128×128 bytes are added the first encoding parity of 14×130 bytes and the second encoding parity of 8×142 bytes; thus, a two-dimensional array as shown in FIG. 8 is formed. In this two-dimensional array, consecutive 16 rows constitute one logic sector, and one logic sector consists of 128×16=2048 bytes. A sector address is added to each row of the two-dimensional array, and a synchronous signal SYNC is further added thereto.
In this manner, the two-dimensional array in which the correcting process using the first encoding parity and second encoding parity can be concluded is constructed. In the disk, recording is carried out on each row from left to right in FIG. 8, and upon completion of the recording, the next recording is carried out on the row located below.
Here, with respect to data arrangements in the optical disk and the optical disk recording and reproducing apparatus, there have been two systems; that is, a continuous servo system in which recording is made in continuous track areas within a physical sector that is a recording and reproducing unit of user data, and a sample servo system in which recording is made in a discrete manner between a plurality of servo areas arranged on a track by using concave and convex sections formed on the disk substrate within the physical sector.
FIG. 9(a) shows one example of a data mapping state on a track in the sample servo system. As illustrated in this Figure, servo fields (SF), which are servo areas, are arranged on a spiral track with constant intervals in a discrete manner. Data are recorded on a data field (DF) between the adjacent servo fields. The physical sector is constituted by a plurality of collected data segments, each of which is formed by a combination of the servo field and data field. Moreover, an address segment containing address information indicating the position of each physical sector is placed in the data field of the leading segment of the physical sector.
FIG. 9(b) is a conceptual drawing that shows the above-mentioned servo field. As illustrated in this Figure, pairs of pits P1 and P2 which are arranged with predetermined intervals in a biased manner in the disk radial direction with respect to the center of the track and pits P3 arranged in the center of the track are formed on the disk substrate as concave and convex sections. The pits P1 and P2 are used so as to obtain a control signal (tracking error signal) that is used upon scanning a light beam directed from the optical head of an optical disk recording and reproducing along the center of the track. Based upon a difference in the quantities of reflected light beams from the pits P1 and P2 at the time when the light beam scans the track, the tracking error signal is generated. Here, the control signal (focusing error signal), used for converging the light beam on the disk recording surface and for scanning the surface, is generated by using reflected light beam from a mirror face within the servo field.
The pits P3 are used for obtaining a clock signal for specifying the positions of the pits P1 and P2. Moreover, the clock signal is also used as a reference clock (recording and reproducing clock) for reproducing the address information of the address segment and for recording and reproducing data on and from data segments.
In the optical disk shown in FIGS. 9(a) and 9(b), the servo fields are formed as concave and convex sections on the disk substrate; therefore, in the case when the disk substrate is manufactured by extrusion molding, etc., it is highly possible that degradation occurs in the characteristics of the data field portions adjacent to the servo fields. In particular, in the case of magneto-optical disks in which the control for birefringence of the disk substrate is essential, the edge of each data field in the track scanning direction tends to deteriorate in optical properties as compared with the other portion of the data field, resulting in a possible higher data error ratio at the edge of the data field.
Moreover, in the optical disk recording and reproducing apparatus, a reproduced signal, which is obtained when scanning is made by the light beam along the track, becomes discontinuous at the ends of the data fields. For this reason, in a circuit process for converting a reproduced analog signal to a digital signal, deviations tend to occur in the process related to the edge portions of the data fields as compared with processes for the other portions of the data fields, resulting in a high error rate at the ends of the data fields.
Here, the following description exemplifies a case in which information is recorded on the optical disk shown in FIGS. 9(a) and 9(b) by using the recording method as shown in FIG. 8. In this case, when data that are to be recorded on the above-mentioned portions (ends of the data fields) adjacent to the servo fields are concentrated, for example, on the same column in the two-dimensional array in FIG. 8, the first encoding process in the error-correction encoding operation forms a code system on each column in the two-dimensional array; therefore, in the columns on the data field edge portions on which data are concentrated, there is a higher possibility of failure in correction in the decoding process of the error-correcting operation, as compared with the coding system of the first encoding process on the other columns.
Moreover, for the reason as described above, if there is an incorrectable error remaining in a column on which data (user data) are recorded as shown in FIG. 8, it is highly possible that the error causes malfunction in the host apparatus for processing reproduced data from the optical disk recording and reproducing apparatus.
In this manner, in the conventional optical recording method and optical disk recording and reproducing apparatus, in the case when an optical disk that might have a high data error rate particularly in the field ends is reproduced in an optical disk recording and reproducing apparatus, there is a higher possibility of failure in correction in the decoding process of the error-correcting operation and the resulting possibility of malfunction in the host apparatus.